Use the Ellipse Select tool and (holding the SHIFT key) drag a circle that has a Bounding Rectangle size of 400 x 400 pixels.

IMAGE > Crop to Selection

Set your Secondary Colour to completely transparent.

IMAGE > Canvas Size: Select By Percentage with a setting of 125% (ensure the Anchor is set to 'Middle')

You should now have a circle that is perfectly centred on your canvas.

Set your Primary Colour to Dark Green - I used Hex: 00660A - and your Secondary Colour to White.

Use the Magic Wand tool to select your white circle and use the Gradient tool, set on Radial, and use the right-mouse button to draw a gradient starting about 1/3 from the bottom of the circle up to the top.

Duplicate the Background layer twice. Call the top layer 'Top' and the middle layer 'Middle'.

Start with the 'Top' layer and choose the Move Selection tool (your circle should still be selected) and grab the top of your circle selection and begin dragging down until the height of the elipse is 350px.

Press Delete.

Go to the 'Middle' Layer and use the Magic Wand to select the area outside the circle and CTRL+I to invert selection.

Choose the Move Selection tool again and grab the top of the circle and drag down until the height of the elipse is 250px. Delete.